Processing used motor oil has been a problem in search of a solution for over fifty years. It is a problem both in size and technology. In the USA, over one billion gallons of used motor oil is “produced”. Little of it is recycled or used effectively and much is improperly dumped. Re-refining is a problem because the very additives which make modern lubricating oils stick to metal surfaces in an engine greatly complicate recovery of the lubricant boiling range hydrocarbons, at least recovery using commercially viable technology. The state of the art of producing, collecting and re-refining of used motor oil and other industrial oils will be reviewed along with some limitations encountered in use of re-refined oil as e.g. a diesel blending component.
Automotive and many industrial lubricating oils are usually formulated from paraffin based petroleum distillate oils or from synthetic base lubricating oils. Lubricating oils are combined with additives such as soaps, extreme pressure (EP) agents, viscosity index (VI) improver, anti-foamants, rust inhibitors, anti-wear agents, antioxidants, and polymeric dispersants to produce an engine lubricating oil of SAE 5 to SAE 60 viscosity.
After use this oil is collected from truck and bus fleets, automobile service stations, and municipal recycling centers for reclaiming. This collected oil contains organo-metallic additives such as zinc dialkylthiophosphate from the original lubricating oil formulation, sludge formed in the engine, and water. The used oil may also contain contaminants such as waste grease, brake fluid, transmission oil, transformer oil, railroad lubricant, crude oil, antifreeze, dry cleaning fluid, degreasing solvents such as trichloroethylene, edible fats and oils, mineral acids, soot, earth and waste of unknown origin.
Reclaiming of waste oil is largely carried out by small processors using various processes tailored to the available waste oil, product demands, and local environmental considerations. Such processes at a minimum include partial de-watering and coarse filtering. Some more sophisticated processors may practice chemical demetallizing or distillation. The presence of organo-metallics in waste oils such as zinc dialkylthiophosphate results in decomposition of the zinc dialkyldithiophosphate to form a carbonaceous layer rich in zinc and often other metals such as calcium, magnesium and other metals present as additives and thus is difficult if not impossible to process. The carbonaceous layer containing the various metals forms rapidly on heated surfaces and can develop to a thickness of more than 1 mm in 24 hours. This layer not only reduces the heat transfer coefficient of tubular heaters rapidly, it also results in substantial or total occlusion of these tubes within a few days.
Successful reclaiming processes require the reduction of the organo-metallics (or ash) content to a level at which the hot oil does not foul heated surfaces. Such reduction can be carried out by chemical processes which include reacting cation phosphate or cation sulfate with the chemically bonded metal to form metallic phosphate or metallic sulfate. U.S. Pat. No. 4,432,865 to Norman, the contents of which are incorporated herein by reference, discloses contacting used motor oil with polyfunctional mineral acid and polyhydroxy compound to react with undesired contaminants to form easily removable reaction products. These chemical processes suffer from attendant disposal problems depending on the metal by-products formed.
Ash content can also be reduced by heating the used lubricating oil (ULO) to decompose the organo-metallic additives. Direct contact heating of ULO with a recycled bottoms fraction was disclosed in U.S. Pat. No. 5,447,628 to Harrison, et al., the contents of which are incorporated herein by reference. The ULO was added to a lower section of a vacuum column with an enlarged bottom section. There was enough volumetric capacity below the first tray of the column to provide “a residence time of 10 to 120 minutes.” The EXAMPLE reported that a residence time of 45 minutes and a relatively constant temperature of 660° F. A dehydrated ULO fraction was mixed with a recycled bottoms fraction in the ratio of 1:45. The long residence time and high temperature were believed sufficient to decompose the additives in the ULO so that a bottoms fraction from this column could be sent to a fired heater to supply the heat needs of the process. The patentee reported that additive decomposition began at 400° F. The Figure in the patent showed that zinc compound decomposition was a function of temperature with time temperature decomposition profiles presented for 400° F., 500° F., 750° F. and 1000° F.
UOP's Hy-Lube process described in U.S. Pat. Nos. 5,244,565, 5,302,282, and many more patents uses a hot circulating hydrogen rich stream as a heating medium to avoid deposition of decomposed organo-metallic compounds on heating surfaces.
The problem of fouling of heated surfaces can be ameliorated to some extent by gentler heating. Some processes such as the fixed bed version of catalytic cracking, the Houdry process, used a molten salt bath to provide controlled somewhat gentle heating of vaporized liquid hydrocarbon passing through tubes of catalyst immersed in the salt bath. Molten metal baths have also been used as a convenient way to heat difficult to process substances to a control temperature e.g. flammability of some plastics is tested by putting a flask with plastic into a bath of molten metal. Use of molten salt bath or molten metal bath or condensing high temperature vapor could be used to reduce uneven heating of heat exchange surface and thereby reduce AT across a metal surface and perhaps slow the fouling of metal surfaces in ULO service, but the additives in the ULO would still tend to decompose on the hottest surface which would be the heat exchanger tubes.
In U.S. Pat. Nos. 7,150,822 and 7,241,377, Malone taught use of a molten metal or molten salt bath for direct contact heating of ULO. The process effectively heats ULO without fouling the heating surface, a molten metal or salt bath, but the process requires a large heavy molten metal vessel for processing of the oil. Start-up of such a process or perhaps operation may have encountered problems as the first commercial unit is believed no longer be in operation.
Solvent extraction with light paraffin solvents such as propane, butane, pentane and mixtures thereof have been practiced by Interline and others. Details of the Interline Process are provided in U.S. Pat. Nos. 5,286,380 and 5,556,548. While the extraction approach seems like an elegant solution to the problem of processing ULO, the process may be relatively expensive to operate. Their quarterly report of May 15, 2002, reports that “It has become evident that demanding royalties based on production is impractical in many situations and countries. Unless and until the re-refined oil produced in a plant can be sold at prices comparable to base lubricating oils, collecting royalties based on production will be difficult. This reality was experienced in Korea, where the royalty was terminated for the first plant, and in England where the royalties were reduced and deferred until the plant becomes profitable.”
Another approach to ULO processing was direct contact heating of the ULO with steam or a non-hydrogenating gas. This solved the problem of zinc additive decomposition fouling of hot metal surfaces by ensuring that the metal surfaces holding the ULO were always relatively cool. The hottest spot in these ULO process was the point of vapor injection. Decomposing additives had only themselves upon which to condense.
A vapor injection ULO process was disclosed in U.S. Pat. No. 6,068,759 Process for Recovering Lube Oil Base Stocks from Used Motor Oil and U.S. Pat. No. 6,447,672 Continuous Plural Stage Heated Vapor Injection Process for Recovering Lube Oil Base Stocks from Used Motor Oil. The heated vapor was steam, methane, ethane, propane or mixtures. Other variations on the theme of ULO vapor injection are disclosed in U.S. Pat. No. 6,402,937 Pumped Recycle Vapor and U.S. Pat. No. 6,402,938, Vaporization of Used Motor Oil with Nonhydrogenating Recycle Vapor, which are incorporated by reference. This approach used a “working fluid” such as methanol or propane which was heated and mixed with ULO to vaporize lube boiling range components. A lube fraction was recovered as a product and the methanol or propane working fluid either compressed or condensed and pumped through a heater to be recycled to heat incoming ULO.
Another concern with existing technology is finding a profitable market for the re-refined ULO. This is of course highly dependent on the method used to recycle the ULO. If coking is used much of the ULO feed ends up as coke, and there is little distillate boiling range material left and it is of poor quality. If extraction and chemical treatment are used, there is a relatively large amount of distillate liquid produced, but the costs are high.
Vacuum distillation of ULO was used by Emerald Services Inc. to produce a material similar to a virgin lube blending stock. Washington State studied this material for use as a ferry fuel with the fuel being a 50/50 blend of recycled lube oil and Ultra Low Sulfur Diesel or ULSD. The Washington State Department of Transportation reported the “re-refined fuel (blend) does not comply with the EPA requirement. The issue is sulfur content. The sulfur content is five times the allowable limit for use in the Emission Control Area (ECA).” Letter of Lynne Griffith, Assistant Secretary, Ferries Division to the Senate Transportation Committee, Dec. 9, 2014. This letter is noted to make the point that some seemingly obvious uses of re-refined ULO as fuel are not options.
Petroleum refiners have been trying for over half a century to devise a satisfactory way to reprocess used lube oil. No process is known which could be considered a commercial success. Despite the abundance of a potentially valuable waste material, namely the lubricating oil boiling range hydrocarbons trapped in the ULO, most ULO is not re-refined. The “state of the art” of used motor oil processing could be summarized as follows:                Chemical additive and extraction approaches can be used to react with or extract everything but zinc additives, but costs associated with such processes are apparently high as evidenced by little commercial use.        Indirect heating in a fired heater causes rapid fouling of metal surfaces. Using milder heating via a double boiler approach or molten metal heating medium can minimize but not eliminate fouling on hot metal surfaces.        Direct contact heating with high pressure hydrogen may eliminate fouling but requires high capital and operating expenses.        Direct contact heating with a recycled bottoms fraction can still suffer from heater fouling. A stream containing the non-distillable additive package and/or the decomposition products thereof is still sent through a fired heater where fouling can occur.        Direct contact heating with steam or a light hydrocarbon “working fluid” vapor is an attractive approach. When steam is the injected vapor, the process can create a water disposal problem and is thermally less efficient because the latent heat of water is lost when the steam is condensed against cooling water or air in a heat exchanger. There are also concerns about possible formation of emulsions or corrosive regions in portions of the plant when water is condensing. When a “working fluid” is used for heating e.g. propane, the water problem is largely eliminated, but large volumes of vapor are needed to provide sufficient heat input so costs increase to heat and recycle such vapor streams. The working fluid approach also calls for a somewhat higher capital cost, because higher pressure operation is generally needed to facilitate circulation of the large volumes of working fluid to heat the used lube oil feed.        
We wanted a better approach, one which is simple and reliable and which does little or no harm to the used lube oil fraction. We define harm as thermally cracking the ULO and generating large amounts of reactive intermediate species, many of which contain chlorides.
Brute force heating by recycling a bottoms stream forces at least some of the additive package to end up in the bottoms which go through a fired heater and cause fouling. The brute force approach vaporizes the lubricant boiling range components but can easily degrade the lube components and contaminate them with significant amounts of the breakdown products of the additive package. The recovered lube boiling range components will have significant value as fuel or cracker feed blending component but are generally not suitable for use as lubricant blending stock, at least not without a lot of expensive hydrotreating. Destructive distillation of ULO by spraying it on top of a coker drum decomposes the additive package and leaves it behind in the coke, but the valuable paraffinic lubricant boiling range hydrocarbons are converted to coker naphtha or other reactive and difficult to process fractions. The lube fraction is arguably “recovered” but is no longer remotely suitable for use as a blending component.
Steam injection for heating of ULO would minimize thermal degradation of lubricant boiling range hydrocarbons in the ULO, but the relatively “wet” approach causes concerns about disposal of waste water, emulsion formation and/or plant corrosion. The “pumped vapor” approach using propane or the like eliminates most water problems but requires a more complicated plant to recycle the hydrocarbon vapor. Large molar volumes of injected vapor are needed because of the relatively low molecular weight and low heat capacity of hydrocarbon vapors. Condensation and separation of injected heating vapor and recovered lubricating components are somewhat expensive.
Another limitation of some conventional approaches to re-refining ULO which use vaporization is the residue. When ULO is vaporized, the vapors are generally high quality material which are easy to deal with although frequently contaminated by degradation of the additive package. Even when a gentle enough heating method is used e.g. WFE and a hard vacuum, a significant amount of the ULO must be left behind in the residue fraction to make it pumpable. We want to recover the valuable lube boiling range components, but wanted to minimize the amount of residue while still leaving the residue liquid enough to pump. In practice usually 20 LV % or more of the ULO was left behind in the residue package.
We wanted to vaporize the valuable lube components and/or the readily crackable components in ULO. When use of recovered lubricant boiling range fractions as a lubricant blending stock was contemplated, it was important to recover this fraction without unduly damaging it. When used lube oil was to be “recycled” by feeding it to a catalytic cracking unit, it was not so important to prevent thermal cracking of the recovered lubricant boiling range fraction, but it was essential to do so without fouling the plant heaters used to vaporize the used lube oil. We realized that there was a way to overcome the deficiencies of the prior art process by doing something akin to early treatment of FCC feed. In the FCC process, refiners charge a selected distilled fraction to the cracking unit. A distilled feed is used because distillation leaves behind unwanted metal species which are a poison to the cracking catalyst. Our approach was something like an island hopping campaign in war. We did not care about eliminating the enemy, namely the additive package, if we could get around it.
We found that a superheated distillate boiling range material was the ideal material to use to heat and vaporize the used lubrication oil. The superheated distillate could be recovered from an effluent of the used lube oil refining process. It was also possible, and would be preferred in some instances to use a distillate boiling range material which had never been used as a lubricant fraction. In many cases the ideal superheated distillate boiling range material would be a cat cracker feedstock which was too aromatic to be an ideal FCC charge material. Refiners have known for decades that aromatic material is hard to crack, and they try to limit the amount of aromatics in the fresh feed to the unit and also try to limit the amount of aromatic rich material recycled within the FCC unit. These aromatic materials were hard to process in the FCC unit because the aromatics were very thermally stable. This property—thermal stability—makes aromatic rich materials ideal for use as a superheated fluid for vaporizing used motor oil. When aromatic rich liquids such as a cycle oil or slurry oil from an FCC unit are used for ULO re-refining, the resulting product is somewhat less desirable as a cracker feed because of the high aromatic content. The multi-ring aromatics typically in such FCC streams do not become easier to crack because of the presence of significant amounts of aliphatic lube components, but the mixture still has significant value as cracker feed stock.
There are benefits and burdens associated with different distillates. A recycled recovered lube oil fraction will always be available from a ULO recycling plant so no outside source of feed is required. The only drawback to use of this material is that the highly paraffinic lube stock is readily cracked both thermally and catalytically. When the process is optimized to produce a lube blending stock from ULO, it will usually be desirable to gently heat the recycled lube fraction to avoid thermal cracking. When the process is run to make cracker feed or some other fuel oil product, there is less concern about high temperature in the furnace used to make the superheated fluid so higher temperatures can be tolerated even if there is some thermal cracking.
The key feature is heating the ULO with a superheated distillate vapor. This material can be a recycled product fraction, a portion of the lubricant boiling range product recovered from the used lube oil fractionator. This material is essentially free of metals and could safely be heated in a heat exchanger or fired heater without fear of fouling. This fraction is always available downstream of the re-refining plant. It has a high boiling point which is essentially the same as the boiling point of the lubricating oil components of the ULO feed to the process. This vaporized lube fraction could be readily condensed at high temperature. It also by virtue of its high molecular weight carries a lot of energy with it when heated in a furnace or heat exchanger so that undue volumetric amounts were not required to achieve the desired amount of direct contact heating of ULO feed. An outside distillate stream can also be used and is preferred in some cases. This requires a source of outside distillate material separate from the ULO feed, but such materials are always available. Preferably the outside distillate materials have been through one or more distillation steps so that relatively clean superheated vapor is used to heat and vaporize lubricant boiling range material from the ULO. The high grade energy in the resulting mix of superheated vapor and vaporized distillate boiling range material may be used to effectively and gently preheat incoming ULO liquid feed for energy savings.
The use of a vaporized distillate boiling range hydrocarbon simplified the process flow and gave the option to achieve significant improvement in thermal efficiency of the process and facilitate plant operation. When a recycle fraction of lubricant boiling range material is used no purchased working fluid is needed save perhaps at initial startup. The vaporized lube fraction condenses readily at elevated temperature, even under vacuum conditions so fin fan coolers or heat exchangers can easily condense these vapors. Preferably much of the energy in the vaporized lube fraction is recovered by heat exchange with ULO feed.
The condensed lubricant boiling range material recovered from the product is a relatively stable material at least far more stable than the feed ULO. This stable material may tolerate a lot of heating by heat exchange with heavy residue material withdrawn from the product fractionator, by heating in a fired heater or by heat exchange with some other high temperature heat source. While the paraffinic lubricant boiling range material is subject to thermal cracking, at least the only cracking products will be those typically experienced during mild thermal cracking of relatively pure hydrocarbons—a modest amount of olefins. Such a stream is a valuable product, especially as a lubricant blending stock. With hydro-processing it can produce a premium lube blending stock or fuel. Such modestly cracked streams with some olefins but few dienes may be processed easily using conventional refinery technology, whereas severely cracked streams with high diene content require extra and expensive processing to make them stable enough for further processing.
When a distillate material not derived from ULO is used as the source of superheated fluid, it must be brought in from outside. Such materials will usually have a significantly higher aromatic content than any lubricant fractions. The outside material may be safely heated by heat exchange with various ULO re-refining plant streams with few concerns about thermal cracking and may also be heated in a conventional furnace or fired heater with greatly reduced concerns about thermal cracking when compared to a more paraffinic lubricant based material.